Environ. Sci. Technol. 1994, 28,474-478
Removal of Cu(I1) from Aqueous Solution with High Ionic Strength by Adsorbing Colloid Flotation Cheng-Shlun Lin and Shang-Da Huang' Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan 30043, Republic of China
Copper ion was removed from wastewater by adsorbing colloid flotation with iron(II1) hydroxide and sodium dodecyl sulfate (SDS) at optimum pH (7.5-8.5) provided that the ionicstrength of the solution was small (containing no greater than 0.03 M Na2S04 or 0.5 M NaN03). The electrolyte tolerance of adsorbing colloid flotation of Cu(11) is improved with the aid of activators, such as Mg(I1) or Ca(I1) ions. Effective separation was achieved over a greater range of pH (7.5-10.0) from wastewater with much greater ionic strength (0.8 M Na2S04 or 1.6 M NaN03) when a mixture of SDS and sodium oleate is used as the frother and collector, in contrast to that using SDS alone. The applicability of an adsorbing colloid flotation technique to remove heavy metals from wastewater is thus greatly extended. As the separation efficiencyis insensitive to the variation of pH and ionic strength, the flotation process operates much more easily with a mixture of SDS and oleate as the frother and collector.
Introduction Numerous techniques exist to remove heavy metal ions from wastewater. The most common method is chemical precipitation (generallywith lime or NaOH). This method may be costly: it requires relatively large amounts of space for the clarifier; it typically produces a wet, bulky sludge; and it generally requires final filters for polishing if small residual levels of metals are desired. Other available processes include ion exchange, reverse osmosis, adsorption on activated carbon, and solvent extraction. These methods are relatively expensive, involving either elaborate and costly equipment or high costs of operation and energy requirements. The ultimate disposal of the contaminants may also be a problem with some of these techniques. Foam separation processes have been utilized to separate or to enrich minerals, surfactants, proteins, enzymes, microorganisms, and various metallic and nonmetallic ions (1). The extensive literature in this field has been reviewed by Lemlich (1-5), Somasundaran (6, 7 ) , Grieves (&IO), Sebba (II),Mizuike (12,131,Clarke and Wilson (14,151, Caballero et al. (16),and Huang (17). These techniques are based on the fact that surface-active material tends to concentrate at the gas-liquid interface. When air is bubbled through the solution, the surface-active material adsorbs at the surface of the rising bubble, which then separates it from the solution. The substance to be removed (i.e., colligend), if not surface active, can be made surface active through the union with or the adsorption of a surface-activematerial. For instance, adsorbing colloid flotation involves the addition of a coagulant [alum or iron(II1) chloride] to produce the floc. The dissolved material is adsorbed onto the floc particle and/or coprecipitated with it. A surfactant is then added, it adsorbs onto the floc particle and renders it hydrophobic, and the
* To whom correspondence should be addressed. 474
Environ. Sci. Technoi., Voi. 28, No. 3, 1994
floc (with adsorbed metal ions) is removed by flotation. The surfactant added is called the collector. To treat dilute waste, foam separation techniques appear to possess distinct advantages: small energy requirements, small residual concentrations of metals, rapid operation, small space requirements, flexibility of application to various metals at various scales, production of small volumes of sludge highly enriched with the contaminant, and moderate cost (14,151. The chemical costs and capital costs of wastewater treatment by adsorbing colloid flotation have been estimated and compared with those of lime precipitation (14, 18); economics appear to favor adsorbing colloid flotation by a substantial margin. Foam separation has one distinct disadvantage: the separation efficiency decreases with increasing concentration of inert salt in solution. Doubly-charged anions, such as sulfate, have been found to decrease the efficiency of the separation to a greater extent than singly-charged anions (such as chloride and nitrate) (14,15,17-23). This drawback restricts the applicability of foam separation techniques for wastewater treatment and is probably a major reason why these techniques are not commonly used for wastewater treatment, despite their numerous advantages. Foam separation of copper has been extensively investigated. Rubin et al. (24) showed that copper was separated by ion flotation or precipitate flotation with sodium dodecyl sulfate (SDS). Rubin and Johnson (25)examined the effect of pH on the ion and precipitate flotation of copper and copper(I1) hydroxide with SDS or stearylamine as the collector. Pearson et al. (26) removed copper(I1) hydroxide by flotation with a diamine diacetate of large molar mass as the cationic surfactant and with l-methylethanol or Dowfroth 250 as the frothing agents. Kim and Zeitlin (27)collected copper in seawater by adsorption colloid flotation using Fe(OH)3 and dodecylamine. Okamot0 and Chou (28) removed copper by foam separation with a chelating surfactant (4-dodecyldiethylenetriamine). Chatman et al. (29) investigated the adsorbing colloid flotation of Cu(I1) from solutions with Fe(OH)3 and SDS; they found that the flotation efficiency was poor at high ionic strength. The optimal pH range for adsorbing colloid flotation of Cu(I1) with Fe(OH)3 and SDS was 7.0-8.5 (22, 29). In this range, over 98% of the copper was removed from the solution. Residual copper concentration is high at smaller pH (pH < 6.5), presumably due to incomplete co-precipitation of Cu(OH)2with Fe(OH)3. When the pH value is adjusted to too large avalue (pH 2 9.5),the surface of the floc (primarily a mixed metal hydroxide) becomes negatively changed, which prevents adsorption of the anionic surfactant; hence, the separation is poor (22,29). Allen et al. (30) studied the flotation of copper with iron(111) hydroxide, sodium dodecyliminodiacetate, and a carrier surfactant (SDS and Tween 20, polyoxyethylene(20) sorbitan monolaurate). Pilot plant studies (31) revealed that copper was effectively removed by adsorbing colloid flotation in a continuous-flow pilot plant. Effluent 0013-936X/94/0928-0474$04.50/0
Q 1994 American Chemical Society
Table 1. Effect of pH and Ionic Strength on Separation8
PH
0.0
7.5
0.18 f 0.02 0.09fO.03 0.12f0.01 0.31 f 0.01
8.0
8.5 9.0
residual Cu (pprnbb NaN03 (M) 0.4 1.67 f 0.14 0.9Of 0.13 1 . 2 9 f 0.30 >10
Table 4. Effect of pH on Adsorbing Colloid Flotation of Cu(I1) with Sodium Oleate8 residual Cu (pprn)
0.5
0.6
duration of flotation (min)
9.0
9.5
10.0
10.5
11.0
11.5
2.99 f 0.40 1.06k 0.18 1.44f 0.12
>10 >lo >lo 210
5 10
>10 >10
0.18 0.08
0.12 0.07
0.09 0.06
0.09 0.06
>10 >10
>10
@
a
PH
Sodium oleate = 50 Dam.
SDS = 50 ppm, duration of run = 10 min. Average value f
standard deviation, n = 3.
Table 5. Effect of NaN03 Concentration on Separation Using Sodium Oleate Alone8
Table 2. Effect of Mg(II), pH, and Ionic Strength on Separation8 residual Cu (ppm) Mg(I1) (ppm) 20 30
pH
NaN03(M)
0
10
8.0
0.6
4.01
0.8 1.0
>10 >10 >10
0.6
>10
3.99
0.8 1.0
>10 >10 >10 >10 >10 >10
8.5 9.0
0.6 0.8 1.0
9.5
0.6 0.8 1.0
>10
>10
>10 >10
1.87 4.13 1.21 4.87 1.74 9.10
1.30 2.37 0.67 1.97 1.96 >10
4.56 >10
>10
>10
50b 1.89 f 0.24 2.16 f 0.30 2.33 f 0.57 0.74 f 0.26 1.04 f 0.08 1.39 f 0.47 0.64 k 0.22 0.73 f 0.33 0.87 f 0.29 0.68 f 0.18 1.01 f 0.35 1.80 f 0.25
SDS = 50 ppm, duration of run = 10 min. Average value f standard deviation, n = 2-3. Table 3. Effect of Ca(I1) on Separation from NaNOa Solution (0.6 M)a residual Cu (ppm)b PH
0
50
8.5 9.0 9.5
>10
>10
>10
>10 >10
>10
6.97 f 0.50 3.14 f 0.12
>10
100
200 >10 0.27 f 0.05 0.26 f 0.10
a SDS = 50 ppm, duration of run = 10 min. b Average value f standard deviation, n = 2.
copper(I1) concentrations in the range 0.1-0.3 mg/L can be routinely obtained by means of this technique. Gannon and Wilson (32)investigated the flotation of Cu(I1) with a mixture of sodium dodecyl phosphate (SDP) and hexanol using iron(II1) hydroxide as the floc. Effective separation with residual Cu(I1) concentrations less than 2 mg/L were achieved a t pH 10.0 in the presence of 0.05 M S042-. In addition to SDS, which is the most commonly used collector for adsorbing colloid flotation, carboxylic acids and their salts are the other types of collectors commonly used in mineral flotation. Fuerstenau (33) used sodium oleate as the collector for the flotation of hematite; adsorption of oleate on hematite involved a chemical mechanism [formation of iron(II1) oleate a t the surface] and oleate anions absorbed on hematite even a t a pH two units above the point of zero charge (PZC) of hematite. Peck et al. (34)provided infrared spectral evidence of the formation of iron(II1) oleate on the floc surface. Sodium oleate (or a mixture of SDS and sodium oleate) has been used as the collector for adsorbingcolloid flotation of various heavy metal ions by many investigators (35-
PH
0.0
residual Cu (ppm) NaN03 (M) 0.26 0.4b
10.0 10.5
0.07 0.06
0.07 f 0.01
0.08 f 0.01
7.04 i1.23 >10
0.6
>10 >10
Sodium oleate = 50 ppm, duration of run = 10 min. * Average value f standard deviation, n = 2. @
Table 6. Effect of Surfactant Concentration on Separation from NaN03 Solution (1.6 M)8 sodium oleate (PPd 10 10 10
SDS (PPm) 0 10
20
residual Cu (ppm)b
>10 1.46 f 0.48 0.27 f 0.02
pH = 9.0, duration of run = 10 min. b Average value f standard deviation, n = 2. @
44). However, the benefits of this mixed collector (SDS/ oleate) through its compensation for the deleterious effect of increasing ionic strength on adsorbing colloid flotation seems never to have been explored. We showed that the effect of decreasing separation efficiency by increasing ionic strength of the solution for adsorbing colloid flotation of various heavy metal ions with iron(II1) hydroxide and SDS can be compensated somewhat with the aid of activators such as Al(II1) and Zn(I1) ions (19-22, 45). Here we demonstrate that the less toxic metal ions, such as Ca(I1) or Mg(II), can also be used as activators to remove Cu(I1) from wastewater with high ionic strength. We also show that the electrolyte tolerance of adsorbing colloid flotation of Cu(I1) is improved significantly (from 0.05 to 0.8 M Na2S04) when a mixture of SDS and sodium oleate is used as the frother and collector, compared with SDS alone. The applicability of adsorbing colloid flotation for removal of heavy metals from wastewater is thus greatly extended. Furthermore, effective separation of Cu(I1) can be achieved over an increased pH range (7.5-10.0) from solutions with high ionic strength (0.8 M NazS04) using SDS/oleate as the collector. The co-precipitation of metal ions with iron(111)hydroxide is generally more efficient at larger values of pH to gain more complete separation by using a mixture of SDS and oleate as the collector. In addition, the flotation process operates much more easily as the separation efficiency is insensitive to variations of pH and ionic with the SDS/oleate mixture as the collector.
Experimental Section The batch foam flotation system used was similar to that described earlier (19,21,22). A glass column (length Environ. Sci. Technol., Vol. 28, No. 3, 1994 475
Table 7. Effect of pH on Separation from NaNOs Solution (1.6 M) with Mixture of SDS and Sodium Oleatea
residual Cu (ppm)b duration of flotation (min) 6.5 5 5.91 f 0.54 10 5.40 f 0.41
7.0 7.5 1.04 f 0.01 0.45 f 0.11 1.01 f 0.01 0.43 f 0.12
8.0
PH 8.5
9.0
0.42 f 0.11 0.34 f 0.05
0.36 f 0.03 0.30 f 0.02 0.29 f 0.02 0.27 f 0.02 SDS = 20 ppm; sodium oleate = 10 ppm. Average value f standard deviation.
Table 8. Effect of Sulfate and Al(II1) on Separationa
PH 8.0 8.0 8.0 8.0 8.0 8.0
8.0 8.5 8.5 8.5 8.5 8.5 8.5 8.5 a
Na2S04 (MI
Al(II1) (PPd
0.03
0 0
0.05 0.05 0.10 0.20 0.25 0.30 0.03 0.05 0.05 0.10 0.20 0.25 0.30
20 20 20 20 20 0 0
20 20 20 20 20
residual Cu(I1) (ppm) 0.59 >10
0.11 0.17 0.37 1.97 3.47 0.53 >10
0.08 0.19 0.47 0.99 >10
SDS = 50 ppm, duration of run = 10 min.
9.5
10.0
10.5
0.25 f 0.05 0.24 f 0.03
0.57 f 0.08 0.51 f 0.07
2.98 i 0.63 2.83 i 0.57
Table 9. Effect of Surfactant Concentration on Separation from NazSOd Solution (0.8 M)e
sodium oleate (PP4
SDS (PPm)
residual Cu ( p p d b
10
0
10 10 20 20
10 20 10 20
>10 >10 3.05 f 1.43 1.19 f 0.27 0.43 f 0.08
pH = 9.0, duration of run = 10 min. * Average value f standard deviation, n = 2.
determine the { potential. It is difficult to measure the {potentials of the flocs in solutions with high ionic strength (such as a 0.8 M NaN03 solution); the values of tpotential are used only qualitatively.
Results and Discussion 60 cm and inside diameter 3.5 cm) was used for the flotation. The bottom of the column was closed with a rubber stopper with holes for a gas sparger and a stopcock to take samples and to drain the column, respectively. The gas sparger (pore size 25-50 pm) was commercially available gas dispersion tube. A lipped side arm near the top of the column served as a foam outlet. Compressed air was generated from an air pump. The rate of airflow was adjusted with a needle valve (Hoke) with micrometer control and measured with a soap film flow meter. The air was purified by passage through glass wool to remove particulates, through Ascarite to remove carbon dioxide, and through distilled water to control humidity. The airflow rate was maintained at about 85 mL/min. Sodium oleate (laboratory grade) was used as the collector, and sodium dodecyl sulfate (SDS) was used as the collector and frother. Cu(N03)~3H20, Fe(N03)r9H2O7 A1(N03)3.9H207Mg(N03)r6H207Ca(N03)~4HzO, NaOH, NaN03, and Na2S04 (reagent grade) were used for sample preparation. Synthetic wastewater was prepared from Cu(N03)2.3Hz0, and the ionic strength of the wastewater sample was adjusted with NaN03 or Na2S04. All experiments were run using 250 mL of solution. The initial concentration of Cu(I1) was 50 ppm. The dosage of Fe(111) was 100 ppm for all runs. Measurements of pH were made with a digital pH meter (Radiometer PHM 82). Concentrations of copper were determined with an atomic absorption spectrophotometer (Varian Spectr AA-20). {potentials of particles were measured with a Zeta meter (Zeta-Meter, Inc.), consisting of a cell across which a potential can be applied that will cause the charged particles to move. The period required for a colloid particle to pass a certain distance was measured. A total of 10-20 particles were tracked. The average velocity of the particles is calculated a t a known applied voltage to 476
Environ. Scl. Technol., Vol. 28, No. 3, 1994
Adsorbing Colloid Flotation of Cu(I1) with SDS and Activators. The effect of ionic strength variation (adjusted by adding NaN03 salt) and pH on the separation efficiency is shown in Table 1. The separation efficiency decreases with increasing ionic strength, presumably due in large part to the decreased {potential of the floc at high ionic strength (21, 22, 45). Effective separation with residual copper levels less than 3.0 ppm was achieved provided that the concentration of NaN03 in the solution was no greater than 0.5 M in a pH range of 7.5-8.5. The effects of Mg(I1) (as the activator), pH, and ionic strength of the solution are shown in Table 2. When Mg(11)(20 ppm) was used, effective separation with residual levels of copper less than 3.0 ppm was achieved from a solution containing NaN03 (0.6 M) at pH 8.0-9.0. When the concentration of NaN03 was 0.8 M or greater, or the system a t pH 9.5, a greater Mg(I1) concentration was required to make the floc floatable. When Mg(I1) (50ppm) was added, effective separation was achieved from solutions with relatively higher ionic strength (1.0 M NaN03) and at a greater range of pH (8.0-9.5). This effect may be explained as follows: The surface potential of the floc is less positive (or more negative) with increasing pH or NaN03 concentration of the solution; therefore, a larger amount of Mg(I1) is required to make the surface potential of the floc positive enough for adsorption of an anionic surfactant. Table 3 shows the effect of Ca(I1) as the activator to separate Cu(I1) from the NaN03 solution (0.6 M) at pH 8.5-9.5. The amount of Ca(I1) needed was greater than that of Mg(I1). The reason is presumably that the solubility of is much greater than Mg(OHh, SO that the co-precipitation of calcium species with the floc is not as efficient as that of the magnesium species. When Ca(I1) (200 ppm) was added, effective, separation with residual copper levels less than 3.0 ppm was achieved at pH 9.0-9.5.
Table 10. Effect of pH on Separation with Mixture of SDS and Sodium Oleate in the Presence of NazSOd (0.8 M). residual Cu (ppm)* PH
duration of flotation (min)
7.0
7.5
8.0
8.5
9.0
9.5
10.0
10.5
5
4.57 f 0.81 4.56 f 0.78
2.26 f 0.23 1.49 f 0.12
0.51 f 0.02 0.47 f 0.02
0.47 f 0.09 0.40 f 0.06
0.51 f 0.12 0.43 f 0.09
0.55 f 0.08 0.54 f 0.07
1.54 f 0.45 1.43 f 0.50
>10
10
210
*
SDS = 20 ppm; sodium oleate = 20 ppm. Average value f standard deviation, n = 2. Table 11. Effect of NaNOs, Mg(II), and Sodium Oleate on 5 Potential and Separation by Adsorbing Colloid Flotatione NaN03 (MI
Mg(I1) (ppm)
sodium oleate (ppm)
{potential (mWb
0 0.8 0.8 0.8
0
50
0 0 0
0
50
+39 -47 +36 -8 1
0
residual Cu (ppm)c 0.09 >10
2.16 0.21
All runs were made with Cu(1I) (50 ppm) and Fe(II1) (100 ppm) initially. * All runs were made without SDS,pH = 8.0. SDS = 50 ppm, duration of flotation = 10 min, pH = 8.0.
Adsorbing Colloid Flotation of Cu(I1) with Sodium Oleate. The results for the removal of copper by adsorbing colloid flotation using sodium oleate alone are shown in Table 4. The floc was completely floated, and residual copper levels less than 0.2 ppm were achieved over the range of pH 9.5-11.0. As no stable foam was produced at pH 9.0 or below, the separation was poor under these conditions. When sodium oleate was used for flotation, separation was effective from solutions containing NaN03 concentrations no greater than 0.2 M (shown in Table 5). When the concentration of NaN03 was 0.4 M or greater, a stable foam layer was not formed and redispersion of the floc occurred during flotation. Adsorbing Colloid Flotation of Cu(I1) with SDS and Sodium Oleate. The stability of the foam and the separation efficiency were improved by using a mixture of SDS and oleate as the frother and collector. Table 6 shows the effect of SDS and oleate a t pH 9.0. When SDS (10 ppm) and sodium oleate (10 ppm) were added to the solution, effective separation was achieved, but a stable foam layer was not formed (although the foam layer was more stable than that produced when using sodium oleate alone). With the addition of SDS (20 ppm) and sodium oleate (10 ppm), the floc was completely floated and a stable foam layer was formed. Hence, we used such a surfactant mixture to treat the wastewater containing a greater concentration of electrolyte (1.6 M NaN03). The results are shown in Table 7. Satisfactory separation efficiency was evidently achieved over a broader range of pH, 7.0-10.5. The reason is presumably the chemisorption of oleate on the floc surface (33, 34, 46, 47). When chemically adsorbing surfactants are used as collectors, the flotation behavior is not significantly affected by the surface charge of the floc; hence, we can achieve good separation efficiency a t high pH and high ionic strength. SDS is used as the frother to maintain a stable foam. Note that the activators Mg and Ca are not needed in the SDS/ oleates system. We compared the separations of copper(I1) from solutions containing NazS04 usingAl(II1) as the activator (and SDS as the collector) with that using a mixture of SDS and oleate. As shown in Table 8, the separation was
effective provided that the concentration of Na~S04was no greater than 0.25 M with the aid of Al(II1) and SDS. When a mixture of SDS (20 ppm) and sodium oleate (20 ppm) was used (Tables 9 and lo), good separation was achieved a t much greater ionic strength (0.8 M NazS04) and a broader range of pH (7.5-10.0). Effects of Ionic Strength, Activator, and Collector on { Potential and Flotation Efficiency. The effects of NaN03, Mg(II), and sodium oleate on the separation efficiency of Cu(I1) by adsorbing colloid flotation were correlated with the {potential of the floc (shown in Table 11). The floc is primarily a mixed metal hydroxide that bears a charge dependent on the relative amount of each metal ion, on the concentration of each electrolyte, and on the pH of the solution. The { potential of the floc was positive in the solution with pH at 8.0 if no NaN03 was added, and an effective separation was achieved using SDS alone without an activator [such as Mg(I1)I or other collector (such as oleate). The { potential of the floc became negative when NaN03 (0.8 M) was added, and the floc was not removed by flotation with SDS, presumably because the anionic surfactant SDS was not adsorbed onto the negatively-charged floc surface. However, when Mg(11)(50 ppm) was added, the {potential of the floc became positive, and an effective separation by flotation was achieved. The effects of ionic strength and Mg(1I) on foam flotation of the floc are well explained by their effects on the tpotential of the floc. When sodium oleate (50 ppm) was added to the solution containing NaN03 (0.8 M), the { potential of the floc became more negative, which indicated that oleate was adsorbed onto the negatively charged floc surface (presumably due to chemisorption), such that the surface of the floc became hydrophobic; therefore, the floc was removed effectively from the solution by flotation. We are currently trying to improve the electrolyte tolerance of adsorbing colloid flotation of various heavy metals (Pb, Cd, Cr, and Zn) by using the mixture of SDS and oleate as the frother and the collector. The preliminary data show that this approach is promising.
Acknowledgments We thank the National Science Council of the Republic of China for a grant (NSC 82-0421-M-007-095-2).
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Received for review June 14, 1993. Revised manuscript received November 12, 1993. Accepted November 18, 1993.' Abstract published in Advance ACS Abstracts, December 15, 1993.
